The present invention relates to a load impedance detector for detecting a variation of the load impedance of a circuit, particularly, an audio power amplifier, and further relates to a power supply for the audio power amplifier employing the impedance detector for controlling the supply voltage in response to a variation in the impedance of one or more loudspeakers.
In an audio power amplifier which drives one or more speakers arranged to be switched on or off during record playback, it is desirable that the power supplied to such speakers be maintained constant despite a varying load impedance. If a single speaker is employed, a replacement of the speaker with one having a different impedance value would require that the output power to the speaker be manually switched so that the impedance difference is compensated. In applications where plural speakers are connected to the power amplifier through a switched network, an automatic power control circuit would require a relay for switching the output power level if the switched condition imposes a different load impedance.
One approach would be to employ a bridge-circuited detector having the load impedance connected in one arm of the bridge. However, the reactive component of the load impedance causes an ambiguity in the detector so that it tends to falsely distinguish the desired impedance value.